1. Field of the Invention
The present invention relates to a multilayer flexible printed circuit board that is connected to a circuit substrate and a device to transmit signals, and an electronic device including such a multilayer flexible printed circuit board, and particularly to a multilayer flexible printed circuit board that can provide a high shielding effect for transmitted signals and an electronic device including such a multilayer flexible printed circuit board.
2. Description of Related Art
Flexible printed circuit (FPC) boards are used to connect circuit substrates or to connect a circuit substrate and a device such as a disk drive, display panel or memory device. The flexible printed circuit boards include a laminate in which a conductor layer made of a patterned copper foil or the like is laminated on a base material made of an insulating resin such as polyimide. In recent years, in order to handle the increased types of signals transmitted via the flexible printed circuit board and the miniaturization of devices in which the flexible printed circuit board is used, multilayer flexible printed circuit boards including a plurality of conductor layers laminated with insulating layers interposed therebetween have also been widely used.
With such a multilayer flexible printed circuit board, a technique has been used in which, rather than using one of the conductor layers as a wiring layer for transmitting signals, a pattern is formed in a planar form to have the same potential and the layer is used as a ground layer having the ground potential, and thereby a shielding effect that prevents the influence of noise on the transmission signals transmitted via the flexible printed circuit board can be obtained.
FIG. 7 shows an example of a conventional multilayer flexible printed circuit board disclosed in JP H9-251813A.
In a cable 500 for transmitting record and playback signals that is a conventional multilayer flexible printed circuit board shown in FIG. 7, a shielding member 502 serving as a conductor pattern is formed on a resin base member 501, and signal wiring 504 is formed on the shielding member 502 via an insulating layer 503 made of an insulating member. By grounding the shielding member 502, the conductor layer is caused to exhibit a mirror image effect, and the inductance of the signal wiring 504 as a round-trip signal line is reduced, so as to achieve a higher data transfer speed and reliable transmission of high frequency signal components.
By providing a conductor layer in a multilayer flexible printed circuit board and grounding it to form a ground layer as described in the technique disclosed in the patent document mentioned above, it is possible to obtain the effect as a shielding layer of the signal wiring disposed in the signal wiring layer of the multilayer flexible printed circuit board.
However, in order to obtain sufficient shielding effects for the signals transmitted via the signal wiring formed in the signal wiring layer of the multilayer flexible printed circuit board, it is important to ground the potential of the ground layer reliably to the ground potential (0 V). In other words, it is necessary to configure a multilayer flexible printed circuit board that is inherently capable of deformation such that the potential of the ground layer does not vary and is always maintained at the ground potential (0 V) even if the multilayer flexible printed circuit board is deformed.